MDD clocked SerialPackager models fail to translate

[bthiele]

The Modelica Device Drivers examples using clocked SerialPackager blocks fail.

I have committed two failing test cases into the testsuite (​https://github.com/OpenModelica/OpenModelica-testsuite/pull/644/commits).

For example, model Modelica_DeviceDrivers.ClockedBlocks.Examples.TestSerialPackager fails with the error message

"Symbolic Error.
Too many equations, over-determined system. The model has 100 equation(s) and 99 variable(s)."

Dymola's diagnosis says: "The DAE has 110 scalar unknowns and 110 scalar equations" and the model simulates.

[lochel]

It seems that the model is already overdetermined when it reaches the backend. So probably something goes wrong while flattening.